Method of labeling



June-24,1969 R. E. FEARON 3,451,778 I f METHOD OF LABELING Filed July 6,1965 Sheet of 2 R E FEARON INVENTOR.

June 24, 1969 R. E. FEARON 3,451,778

' METHOD OF LABELING Filed July 6, 1965 Sheet 2 0:2

R E FEA/PON INVENTOR.

" BY war United States Patent 01 ice 3,451,778 Patented June 24, 19693,451,778 METHOD OF LABELING Robert E. Fearon, 530 S. Lewis Ave., Tulsa,Okla. 74104 Filed July 6, 1965, Ser. No. 469,678 Int. Cl. G01n 31/00 US.Cl. 23230 6 Claims ABSTRACT OF THE DISCLOSURE The chemical concept oflabeling as it appears in the art has been greatly broadened beyond theordinary meaning of the term as it is used in the English language. Ithas come to include not only those techniques that impart visible andoutward evidences that are immediately and directly observable by thesenses, but labeling, as the art has developed, now includes also avariety of procedures for imparting more or less obscure properties,ones requiring observation by means of refined instrumental techniques.As an example of the modern development of the art I refer to my US.Patent No. 3,013,958, entitled Isotopic Labeling. In this, thedifference, that is imparted, and used for the identification ofmaterial, is the substitution of an atom of the same element in amolecular structure, but with a special isotopic choice of thesubstituted atom. Since in this previous art I do not limit myself toradioactive isotopes, but include labeling with stable isotopes as well,it is clear that the property of matter to which my prior labelinginvention applies is indeed an obscure one. The only requirement which Iimpose is thatthe isotope by which labeling is accomplished must be rarein nature and easily recognizable.

I have now discovered a new form of labeling, a form essentially relatedto a characteristic other than isotopy. A requirement for labeling, byanalogy with the teaching of my previous invention, is that thecharacteristic by which the labeling is accomplished must not be acommon property of matter, but must be something rare in nature. I havesucceeded in finding a quality suitable for identification, one usableand easily recognizable which not only is rare in nature, but in fact,does not exist at all, so far as is known. I label materials by admixingwith them very small amounts of chemical compounds of rare gases, andthereafter observe a property of the labeled material due to the amountof such rare gas compound. Included in the rare gas compounds which Iemploy are the chemical combinations of the element xenon, includingespecially the perxenates, the xenic acid derivatives, the oxyfluoridesof xenon, and the complex fiuo-xenates, and any other stoichiometricchemical compounds of xenon which may prove convenient.

I also employ clathrate compounds of any of the rare gas elementssusceptible of this status of chemical combination. Moreover there havebeen predicted theoretically (and band spectra have been observed)diatomic electrically charged ions including such elements as argon,neon, and krypton. Theoretical chemists have pointed out the possibilitythat these ions may indeed have a chemical existence in the same sensethat the onium" radicals exist. Thus, it is likely that chemicalcompounds of rare gases patterned along the onium plan will be found toexist and will include stoichiometric compounds of rare gases other thanxenon and radon. Such chemical compounds, if any be found to exist, alsofall within the purview of my invention, and their uses are contemplatedherein, for reasons which will shortly appear. Clathrates of carbontetrafiuoride, methane, dichloro-difluoro methane, and other gases areusefully employed in my method also.

The basic requirement of labeling in the modern state of the art, andset out in its broadest terms, is that a property or a peculiarity ofsome kind be imparted in a form that is useful for labeling purposes. Tobe useful for labeling purposes such property must be one exceedinglyrare in nature, and one which is capable of being readily and definitelycharacterized to the exclusion of other properties of matter, so thataccurate labeling identification is accomplished. In keeping with themodern concept of labeling, I have found that the addition of compoundsof rare gases imparts properties that are absolutely unique, propertiesthat have a maximum labe1 ing desirability. The reasons are easy tounderstand. First of all, chemical compounds of rare gases, whatevervariety of compound, do not exist at all in nature. To the extent ofchemical knowledge at this time, no such compounds have ever beenobserved in nature by any chemist, or discovered in any chemicalanalysis of natural raw materials anywhere in the world, from thebeginning of time down to this moment. a

It is worth noting that up to about three years ago it was not seriouslyconsidered that there were, or ideally ever could be, any chemicalcompounds at all derived from the rare gases. The chemistry books of tenyears ago characterize this group of elements as having zero valence,and being entirely incapable of chemical combination. The reason for theabove situation is a curious one, and it is important at this point tounderstand it. Xenon, for example, has a hexafluoride which can becrystallized. Xenon hexafluoride is not successfully made by heatingxenon and fluorine in an autoclave and cooling the products. Theequilibrium amount of xenon hexafluoride recoverable in this manner isnegligible, if indeed, not absolutely zero. Xenon hexafluoride, so itappears, exists in equilibrium at an elevated temperature. At theelevated temperature the equilibrium is reversible, and the reaction isvery incomplete. In being slowly cooled to room temperature, the systemof xenon plus fluorine passes through a series of states correspondingwith less and less xenon hexafluoride in equilibirum. The techniquewhich has been found effective to produce xenon hexafluoride from theequilibrium mixture at high temperature involves rapidly chillingportions of the high temperature mixture, producing a temperature changethat is too rapid to allow chemical equilibrium to be maintained duringthe cooling process. In this way xenon hexafluoride derived from thehigh temperature mixture is saved at the low temperature to an extentsufficient that crystals of it can be recovered by condensation from thevapor.

Xenon hexafluoride is important in my labelling art since it is thestarting point for making many chemical compounds of xenon. Thehexafluoride obtained generally in the manner described is a trulydistinct chemical combination of the element, having characteristicsthat would be expected of a hexafluoride of an element in its generalvicinity in the periodic table. The hexafluoride of xenon does notattack glass or silicates at room temperature. It sublimates, as do manyof the fluorides of elements which are nearby in the periodic table.Reacting with water and with other highly polar or ionic Solvents itproduces a variety of derivatives in which the xenon is combined withoxygen and hydrogen, with oxygenhydrogen and metals, or with oxygen,hydrogen, and

complex radicals of one kind or another. The chemistry of xenon hasbecome so extensive and complex at this time that the University ofChicago Press has published a book on this (Malm, J. G., Holt, B. D.,Bane, R. W., Noble-Gas Compounds, H. H. Hyman, ed., p. 167, Universityof Chicago Press, Chicago, 1963). The purpose of this precedingdescription is to clearly set out the fact that there exists the meansof practicing my method.

Particular advantages attach to the employment of rare gas compounds fortracing and labeling purposes. Among the important factors that favorthe use of these compounds is their nontoxic character. Thenon-radioactive rare gases themselves (which are promptly produced fromthe compounds in many chemical environments) are entirely lacking inpoisonous characteristics. Other chemical degradation products whichcome from rare gas compounds are ordinarily derived in such smallquantities that they are not important. Very small amounts of hydrogenperoxide, ozone, water, and the like may be produced. Also, in compoundswherein rare gas radicals have been united to other chemical entities ofa complicated nature, free radicals are produced. These shortly dimerizeto make predictable products, or degrate in ways that are entirelyforeseeable. Since it is the rare gas constituent alone that identifiesa compound of the kind I employ in my tracing, I am at liberty to choosecompounds in such a way that all the degradation products are harmlessto man in the concentrations which I produce.

Another advantage of the rare gas tracers, as has been pointed out, isthe uniqueness of molecules containing rare gases, in view of the factthat these molecules do not occur in nature. This advantage permits theimmediate employment of any method of distinctively recognizing themolecules of rare gases. I apply new methods to the recognition of raregas tracers whenever such new methods become available. Thus, my methodof labeling by using compounds of the rare gases continually becomesmore valuable as a consequence of the extensive efforts being devoted atall times to the recognition and analysis of molecules.

A further advantage of the use of rare gas compounds is in the fact thatsome of the rare gases are indeed very scarce in nature, making thedistinctive recognition of gaseous products of decomposition quitefeasible. Because of the uniqueness of the molecules containing raregases and because of the rarity of some of the gases in nature,extremely low concentrations of these materials can serve for labelingpurposes. Very frequently concentrations less than one part per millionare useful.

It is an object of my invention to provide convenient and elfectivechemical labeling means for identifying and distinguishing products thatmay appear in commerce; such as, textiles, pharmaceuticals, dyes,chemicals, etc.

It is an object of my invention to provide means as recited above inwhich the added labeling material is harmless, compatible lWith thematerial to which it is added, and low in concentration to such a degreethat the cost of the labeling is slight.

It is an object of my invention to provide means and methods forrecognizing the labeling constituents when these are present in materialwhich I have labeled.

It is an object of my invention to provide a method of distinctivelyrecognizing a particular material which has been labeled, distinguishingit from other material which has been similarly labeled and to provide amethod for determining when the labeling was done.

It is an object of my invention to provide chemical labeling of dyes,drugs, plastics, textiles, etc., in a form such that neither thelabeling additive nor its degradation products is appreciably toxic toman or animals.

It is an object of my invention to provide chemical labeling methods,and materials, such that less than one part per million of labelingadditive is required to produce eifective identification of the labeledproducts.

It is an object of this invention to provide means, involving the use ofxenon compounds, whereby the operator can determine the averagetemperature at which a preparation has been stored, and for how long itwas kept at such temperature.

Since the rare gas compounds other than the fluorides are even moreimpossible to synthesize by any direct chemical combination, it is notsurprising that these compounds are not found in nature. Because of itsentire absence in nature, any compound of a rare gas, when added tosomething else, imparts entirely new and distinct properties to thematerial to which it has been added. The new properties thus impartedare attributable exclusively to the rare gas compound. They include thecharacteristic by which many such compounds continuously evolve smallamounts of the rare gases which they contain. Such is the case with thestoichiometric crystallized compounds of xenon. The clathrates alsoevolve rare gases entrapped in their molecular structures, doing thisaccording to an easily predictable law of nature, one which can bearrived at very readily from the standpoint of statistical mechanics.Thus, it is a property of any material labeled by a content of rare gaslabeling substances that, when stored in a closed bottle, it will, aftera time, exhibit a concentration of the rare gas substantially higherthan the ambient level at which such gas is found in air. If a beam ofresonant radiation derived from a sharp spectral line of the same gas bedirected through the space in the top of the bottle containing such alabeled material, opacity to the resonant radiation depends criticallyupon the concentration of the rare gas that has been evolved into thespace by slow decomposition of the rare gas tracer.

FIGURE 1 shows a system adapted to produce resonant radiation, andobserve the absorption of the resonant radiation by rare gas atmospheresoccurring in the vicinity of material labeled by a rare gas compound.

In FIGURE 1 I illustrate the production of pure resonant radiation andthe procedure for directing it through an atmosphere which may containthe rare gas from which the resonant radiation is derived. In thisfigure I produce the resonant radiation by an electrical dischargethrough the gaseous atmosphere 4 contained in the glass envelope 5. Theelectrical discharge occurs between electrodes 3 and extends throughcapillary space 6. The atmosphere 4 may contain xenon at a pressure ofapproximately one micron Hg absolute. The electrons 3 are energized bythe battery 1 and the current is limited by the resistance 2. Radiation7 from the capillary space 6 passes through converging lens 8 and isbrought to focus at 11 after passing through the transparent wall of thebulb 9. The focus 11 is in the interior space 10 of the bulb 9 whichcontains a low pressure gaseous atmosphere which, for example, may bepure xenon at a very low pressure. It is necessary that the xenoncontained in the bulb be at a very low pressure for otherwise the puremonochromatic resonant radiation 12 re-emitted from the xenon atmospherein the vicinity of the focus 11 is absorbed in the atmosphere before itcan escape from the bulb. The pure monochromatic resonant radiation 12passes through the converging lens 13 and is converged in the vicinityof the neck 15 of the bottle 14 after which it impinges on the lightsensitive region 17 of the light detection apparatus 16. The electricalindicating signal from the light detecting apparatus 16 is transmittedover the wires 18 to the indicating electrical meter 19. The reading onthe indicating electrical meter 19 is responsive to the intensity of theresonant radiation arriving at 17 and, therefore, indicates, among otherthings, the resonant absorption due to the atmosphere present in theneck of the bottle 14. If the bottle 14 contains a substance comprisinga mixture that includes among other things a chemical compound of xenon,there is an extra amount of absorption observed in the passage of theresonant radiation 12 through the atmosphere in the neck 15 of thebottle 14.

The light source arrangement just described for generating resonantradiation may be replaced by any laser device adapted to produce thesame spectral wave length and frequency, such laser device being thefull equivalent of my light source up to and including the bulb 9 fromwhich the resonant radiation 12 is emitted.

Another useful labeling property of rare gas compounds is the ability ofsuch compounds to evolve heat when they decompose. To identify thepresence of a rare gas compound in a mixture, all that is necessary isto heat the mixture in a thermally insulated region, supplying the heatfrom a constant source of energy such as an electric resistance element,and to concurrently record the temperature rise in the space. Atemperature rise which is essentially linear can be produced andmeasured for all materials except those which contain something thatdecomposes.

To make the curve of temperature rise as exactly linear- 1y as possible,the thermally insulated space may be protected on the outside with athermal guard. The slightest deviation from a linear temperature risecan be observed when two corresponding experiments of this kind areconducted, one with a rare gas labeling substance present and onewithout. To observe the difference, I establish means (such as athermopile) extending between the two insulated spaces. FIGURE 2 shows atechnique for performing this type of thermal analysis.

In FIGURE 2 I illustrate a form of heat detection apparatus forobserving the presence of xenon compounds or clathrate compounds ofother substances. In the illustration of FIGURE 2 as a matter of choiceI have not provided the thermal guard arrangement surrounding the heatedspaces. I have, however, provided a comparative means of measuring thewarmup of two samples, A and 20B, which are alike in all particularsexcept that one may contain a compound of a rare gas element. In FIG-URE 2 the battery 27 provides electrical energy to the heating elements27A and 27B which are connected in series. The battery 27 is connectedto the heating elements 27A and 27B by the wires 28 and by the returncircuit wire which completes the series connection of these heatingelements. The heating element 27A is in the lefthand of two spaceswherein the warmup is to be compared, whereas the heating element 27B isin the righthand space. The two spaces are both insulated from theirenvironment by being enclosed in a Dewar vacuum 21 contained in thetoroidal glass device 20. For the purpose of decreasing the gain or lossof thermal radiation, the surface 22 is silvered. The glass apparatus isunderstood as a section through a three dimensional structure havingapproximate cylindrical symmetry about the axis AA'. Insulation material22A is present in three places, as is indicated by the stippling.Thermopile junctions adapted to compare the heat evolvement in the twospaces 22B and 22C are indicated at 23. As long as the temperatureduring the warmup remains equal in the spaces 22B and 22C, thethermopile junctions are isothermal and no electrornotive force appearson the wires 24, or is indicated by the electrical recording unit 26.Should it be that the temperature on one side or the other deviates dueto the evolution of heat by an exothermic chemical reaction such as thedecomposition of a xenon compound, an electrornotive force will beindicated by the thermopile junctions 23 and will result in anelectrornotive force on the wires 24 and will be indicated by therecorder 26.

A useful labeling property of some rare gas compounds is emission oflight when the compounds undergo decomposition. Decomposition of a raregas compound differs essentially from the decomposition of any otherknown compounds in one particular. The rare gas compounds are the onlyones which decompose exothermically while yielding atoms, not molecules,of one of the constituents. The constituent so yielded is the rare gasitself, Quite frequently, because of the energy of the decomposition,atoms of the rare gas thus yielded appear in an excited state from whichthey immediately radiate electromagnetic quanta and return to the groundstate. Accordingly, therefore, a technique for observance of thepresence of rare gas compounds comprises heating a portion of anysubstance suspected to contain these materials in darkness and in thepresence of sensitive light observing equipment such equipment mayinclude, but is not limited to, apparatus employing photomultipliertubes. Also suitable are photo tubes in which collected electric chargeis increased by gas multiplication or some other convenient means ofamplification. Infrared and ultraviolet radiation is included in thismethod of detection, to the extent that ligh sensitive equipment candetect such radiation.

FIGURE 3 shows an arrangement for practicing the recognition of rare gascompounds by heating substances in the dark. In FIGURE 3 I provide anoven 29 equipped with thermal insulation 30 between its inner and outerwalls. The oven 29 is also provided with suitable glass or heatresistant plastic windows 33 adapted to transmit visible and nearvisible electromagnetic radiation. Inside the oven I afford a mirror inthe shape of a portion of an ellipsoid of revolution. The ellipsoid ofrevolution illustrated at 31, and shown in section, is to be thought ofin three dimensions as a surface resulting from the rotation of thisportion of the ellipse about the axis B-B, a line passing through thefoci of the ellipse. A support 32 is situated within the oven andcontains sample material 32A. The sample material 32A is so placed thatit lies in the close vicinity of one focus of the ellipsoid ofrevolution corresponding with the mirror 31. Because of the well-knownproperties of light, a mirror of such shape converges the light emittedfrom one focus, directing it to the other focus. In the vicinity of thesecond focus of the ellipsoidal mirror 31, I place a light detectingelement 35 which communicates to a recording or indicating meter bymeans of electrical wires 36. The front of the oven and the lightsensing device is protected from stray light by a blackened light shield34 which is arranged to fit the front of the oven in such a way as tototally exclude room light. When an electrical signal appears on thewires 36 indicating the presence of visible or near visible radiationgenerated at the sample 32A and focused by the mirror 31, passed throughthe windows 33 and impinging on the light sensitive device 35, Iconclude that there is present in the sample 32A a material of anunusual nature, with a very strong probability that the luminosity iscaused by a chemical compound of xenon in the process of decompositiondue to the heat within the oven 29. In use I continuously raise thetemperature of the oven 29 and observe the emission of light as measuredby the signal on the wires 36 using a recorder (not shown), whichrecords the said electrical signals as a function of time.

Since molecules of rare gas compounds are entirely absent in nature,their addition to anything labels it to whatever extent these moleculeshave properties which are uniquely their own. It is, accordingly, withinthe purview of my invention to consider all properties of moleculeswhich are uniquely related to rare gas compounds. Thus, selectiveabsorption spectra can be identified, such spectra not being common toother molecules. Fluorescence phenomena peculiar to molecules containingrare gases may be used to identify them, to the exclusion of all othermolecular substances. Raman spectra of the rare gas compounds aredistinct and absolutely identifying. Nuclear magnetic resonance existsfor any case in which there is a nuclear magnetic moment making suchphenomea observable for a particular rare gas compound. Xenon 129 and131, being nuclei of odd atomic weight, have nuclear magnetic moments,and recognition of them by nuclear magnetic resonance is, therefore,possible in all xenon compounds. These isotopes can. be distinguishedbecause the gyromagnetic ratios of their nuclei is not equal. Moreover,nuclear magnetic resonance data for compounds of these isotopes showsthe status of their molecular combination as well as the fact that theyare xenon compounds.

The thermal neutron capture cross section of xenon 131 (whichconstitutes over 21% of the naturally occurring isotope) is 120 barnsfor thermal neutrons. For xenon 129, it is 45 barns. The captive crosssection is negligibly small for the other naturally occurring isotopesof xenon. The processes are n-gamma capture, corresponding with thedistinct and unambiguous capture spectra of these isotopes of xenon. Noactivation occurs, for the reason that the slow neutron capture leads tostable isotopes, xenon 132 and xenon 130. Plainly, therefore, everychemical compound of xenon has the above described property with respectto neutrons, for the reason that it contains Xenon 129 or xenon 131, orboth.

Xenon 131 may be isotopically enriched if desired, starting with anysample of naturally occurring xenon, and likewise for xenon 129. Suchisotopic enrichment is easily practiced by using a thermal diffusioncolumn, if the operator desires. (Cyclotron resonance or otherisotope-separation technique may be practiced.) I may then labelpharmaceuticals and textiles distinctly, using, for the labeling, tracesof chemical compounds derived from the xenon having a disturbed isotopicratio.

Neutron gamma analysis (bombardment with thermal neutrons and productionof the distinctive gamma spectrum of xenon 131) shows the presence ofxenon 131, whereas other measurements such as the light emissionmeasurement provided in my FIGURE 3 react to the total amount of xenon,as does the thermodynamic measurement corresponding with the apparatusof FIGURE 2.

The same propositions apply to xenon 129. A specific technique forperforming neutron capture measurements to sense the presence ofisotopes 129 and 131 of xenon by their distinctive cascades of captureradiation is set out in S. ,A. Scherbatskoys US. Patent No. 3,080,482.Scherbatskoys technique represents a convenient and useful procedure forperforming this measurement in a quantitative manner, and enables thedetermination of the concentration of xenon 129, of xenon 131, and ofeach of these in a sample. Scherbatskoys apparatus also may be used todetermine the ratio of concentrations of these isotopes by the inclusionof suitable computing machinery to calculate the ratio of the outputfrom two of Scherbatskoys systems. One such system is set to sense xenon129 only, whereas the other is adjusted to sense xenon 131 only. Theoutput of each of the said systems feed separately into the inputs ofthe appropriate computational machinery for the determination of theratio of these isotopes.

Thus, I can identify and distinctly determine materials labeled withxenon, even though they may be labeled with the same chemical compoundof xenon, by detecting the different isotopic ratios characteristic ofthe respective labeling products. Nuclear magnetic resonance techniques,like the neutron test, are a measure of xenon 131 and xenon 129 only.

Because the frequency spectrum measured in a nuclear magnetic resonancemachine is distinctly different for xenon 129 and for xenon 131, I canmeasure the relative amounts of these isotopes present in any samplecontaining them. To do so I place a sample containing both these oddisotopes of xenon in a constant magnetic field and excite nuclearmagnetic resonance by means of auxiliary coils. The input is arranged attwo frequencies, one being so chosen that xenon 129 responds to it tothe exclusion of xenon 131. The second frequency of excitation is sochosen that xenon 131 responds, but not xenon 129. By suitable frequencydiscrimination, the outputs at the two frequencies are separatelyreceived and measured to indicate separately the concentrations of xenon129 and 131 respectively, as present in the sample.

It is of particular interest to know the interval of time that haspassed since a drug substance was made, for the reason that some drugmaterials undergo deterioration with time, and, therefore, should not beused after they become too old. Also, there are problems in plastics,textiles, and the like, in which it would be advantageous to know thetime that passed since the manufacture of a material in commerce. Notonly materials in commerce (and chemical intermediates) are of interest,but, also, on occasion, it is desirable to be able to determine how longa consumer product has been kept around since it was manufactured. Ihave discovered two techniques for performing this determination of timeusing properties of xenon isotopes as evidenced in their chemicalcompounds or otherwise.

Many perishable drugs have a safe keeping time of the order of a year.Therefore, it can be said that if the drug is less than one-half a yearfrom the time of its manufacture, it is definitely all right. A way ofassuring that any substance is not older than approximately half a yearinvolves preparing a labeling additive consisting of a xenon compoundwhich contains xenon 127 (of the type that descends from the isomerictransition). This sub-species of the isotope xenon 127 is radioactive,having a half-life of 32 days. The passage of six halflives correspondswith 192 days, which is slightly more than half a year. In this intervalof time the radioactivity of a sample which initially contained xenon127 decreases from its original value to an intensity 1 /2% of itsoriginal value. At the end of a year the ratio to the original intensityis roughly 1 to 5,000. Thus, it is apparent that the sensitivemeasurement of the radioactivity of a sample of material originallylabeled with a xenon compound containing xenon 127 enables thedetermination of age and, particularly, permits the user to determinethat the sample is or is not in its range of usefulness. Samples olderthan a year have such a negligible activity that they can be recognizedwithout error.

A technique for enabling age determination over a greater range takesadvantage of the fact that the escape of xenon from clathrate compoundsof this element is isotope sensitive. There are different rates ofescape, for example, if xenon 129 and xenon 131 are included. Thus, if apharmaceutical is labeled with a clathrate compound of xenon containingxenon 129 and xenon 131, I can use the ratio determining procedurespreviously set out as an indication of the age of the preparation. Theescape of xenon 129 from a clathrate is more rapid. Therefore, the ratioof the concentrations of xenon 131 to xenon 129 constantly increaseswith the passage of time. A factor which also enters in, as it happens,is the temperature. A larger weighting factor must be given to timespent at a high temperature. Accordingly, taken together with othermeasurements which determine the age of a preparation, the 131 to 129ratio of xenon isotopes may be employed as a means of determining thetemperature at which a preparation has actually been kept. If thetemperature history is the main object of a labeling determination, Imust then include some conventional age determining technique which isnot temperature sensitive. For example, I can include labeling withtritium (as described in my US. Patent No. 3,013,958) and employconventional tritium age determination techniques to obtain time in amanner that does not require knowledge of the temperature. Having thistime as determined from the tritium age, I then measure the xenon 131 to129 ratio in the same sample, which gives me another age figure that isnot temperature independent. The ratio of the tritium to the xenon ageis then made, using the arbitrary assumption of 20 centigradetemperature. (At 20 I take the xenon age factor of temperaturedependency equal to one.) I compute the quotient of the apparent xenonage to the tritium age determination. This quotient gives thetemperature weighting factor for the xenon age. From a table oftemperature weighting factors plotted versus temperature, prepared forthe given clathrate compound, I ascertain the average keepingtemperature at which the preparation has been stored.

Another technique which I have found desirable involves the use ofclathrate compounds to determine time of storage and temperature duringstorage but employs the characteristics of clathrate compounds in adifferent way. Particularly, I call attention to the fact that stronglybound inert molecules such as carbon tetrafluoride are capable of beingcontained in clathrate compounds. As those familiar with the art areaware, the pyrolysis temperature of carbon tetrafluoride is extremelyhigh. It is, therefore, entirely feasible to assay clathrate compoundsin which carbon tetrafluoride is contained in a cage molecule by usingthe heating process in the same way that it is used for inert gases. Avery satisfactory temperature range exists in which the carbontetrafluoride can be evolved as a gas, but is not destroyed by the heat.

Other extremely stable molecular structures are of interest besidescarbon tetrafluoride. Methane, the pyrolysis of which occurs in thevicinity of 900 C., is sufliciently stable. Morever,dichlorodifluoromethane (known in commerce as Freon) is amply stable formy requirements. Just as is the case with the inert gases contained in aclathrate, so likewise the clathrate compounds of these inert moleculesevolve the imprisoned gaseous constituent in a manner that is adeterminable function of temperature and time. Accordingly, therefore,the assay of a solid which contained clathrate imprisoned Freon, carbontetrafluoride, methane or the like, can carry information concerning thetime and temperature of storage which it has endured. The manner ofrecovery of the information comprises heating the [solid clathrate toevolve the imprisoned gaseous constituents and determining the ratio ofthe gaseous constituents and correlating the ratio found with the ratiowhich existed when the material was manufactured. If 'I employ threeclathrate compounds which have different algebraic formulas for theirrate of loss at various temperatures, two independent ratios can bedetermined. From these data it is possible to uniquely determine boththe time of storage and the temperature at which the storage occurred.

My more refined and more completely general method of employingclathrates, as I have outlined it, escapes entirely the requirement forthe inclusion of a radioactive material for dating the time ofmanufacture. The carbon tetrafluoride, Freon, and related materials areentirely valid and useful for tracer purposes for the same reason thatXeon is. These substances, like xenon, are very rare in nature, possiblyrarer than xenon. Moreover, pharmaceuticals, textiles, chemicalintermediates of most types, plastics, and other substances of commonexperience which it is desired to label do not evolve these materials atall except when I have added a clathrate tracer compound deliberately.Sulphur hexafluoride is another example of a useful material serving inthe same manner as does carbon tetrafluoride.

Considering my above description, together with the earlier examples ofthe use of xenon isotopes in clathrate compounds of xenon for tracing, Ihave been able to discover a general rule which is of help in guidingthe operator who desires to explore the scope of my invention. Thegeneral rule is this, that I seek to use a chemical compound, clathrate,or otherwise, derived from a material which is either gaseous or has avery high vapor pressure at room temperature, and which under ordinarycircumstances is very inert and indestructable chemically. I choose thegaseous or vaporizable material from among substances which are rare innature and easily recognizable. I prepare chemical compounds, clathrate,or otherwise, derived from the inert vaporizable tracer substances. Ithen mix the clathrate compounds so prepared with the substances whichit is desired to identify by tracing. Included in the scope of mydiscovery I may at times go further than merely mixing the compounds ofmy inert vaporizable tracer substances, by attaching these chemicalcompounds to the material being labeled, attaching by a chemical bond.Such attachment of the tracer as a prosthetic group is particularlyfeasible in connection with high polymers, plastics, elastomers, and thelike, but this technique may be extended to other fields as desired. Theattachment of the chemical grouping which carries the tracer substancethrough chemical bonding conveys the advantage that miscibility is nolonger an issue, and that the chance of separation of the tracermaterial from the major portion of the batch being marked is prevented.

Methods of detection of evolved gases, such as carbon tetrafluoride,Freon, sulphur hexafiuoride, and the like are used in forms which alsocan be applied to xenon; and their use for xenon and the other noblegases is envisioned. Among useful methods of detection which I employare mass spectrograph procedures, particularly including those which arepresently in commercial use for leak detection. Equipment exactlyanalogous with that currently employed for helium leak detection, infact, is very valuable. In addition, it separate and condense the gasesor compress them and test by nuclear magnetic resonance. This procedurecan be applied to all the gases which are composed of or contain nucleiwith a magnetic moment. Sulphur hexafluoride meets the requirement forthe reason that fluorine has a magnetic moment. Chlorine similarly has arather large magnetic moment. Besides mass spectrography and nuclearmagnetic resonance, I employ the other spectroscopic methods which havebeen referred to in connection with xenon (with the exception of theresonant radiation technique which does not apply very well tomolecules). Tracing with carbon tetrafluoride clathrates and the likecomprises also detection of the gaseous products (or the substances inthe clathrate compound itself) by nuclear methods, including the neutrongamma process using Scherbatskoys technique, as has been previouslymentioned.

Following the specific principles set out above, the operator willdiscern many useful ways to practice my tracer art, the larger number ofwhich cannot be specifically included herein. All such methods andcombinations of matter, falling within the spirit and scope of myinvention, are included within it whether or not specifically recitedabove.

I claim:

1. The method of labeling a material which comprises adding to suchmaterial a chemical compound of a noble gas, and thereafter sensing withrespect to the material having the thus added chemical compound adistinctive property of said chemical compound.

2. The method of labeling a material which comprises adding to suchmaterial a chemical compound of xenon, and thereafter sensing withrespect to the material having the thus added chemical compound adistinctive property of said chemical compound.

3. The method of determining the apparent age of a material after itsmanufacture which comprises introducing therein at the time ofmanufacture in a trace amount a noble gas" chemical compound thatdecomposes in a fashion operative to alter the isotopic composition ofthe material, and thereafter measuring said isotopic composition, saidmeasurement being directly related to the apparent age.

4. The method of determining the average temperature at which a materialhas been kept since its manufacture which comprises introducing thereinat the time of manufacture in a trace amount a chemical compound whichdecomposes at a rate which is temperature dependent in a fashionoperative to alter the isotopic composition of the material, measuringsaid isotopic composition to obtain the apparent age, measuring the trueage of the same material by use of a radio-activity method, andcomparing the apparent and true ages as determined by the two methods.

5. The method of determining the average temperature of a materialduring an interval of time comprising the steps of adding to suchmaterial immediately before the beginning of such time interval a traceradditive having 1 1 1 2 one property which changes in'a known manner asa funcv References Cited tion of temperatureand time, and a secondproperty which UNiTED STATES PATENTS changes as a function of time only,measuring such said first-mentioned property at the beginning of saidtime in- 2,365,553 12/1944 H111 23230 X t'erval and at the end of saidtime interval, measuring 5 OTHER REFERENCES said second property at thebeginning and at the end of said time interval, and determiningthe ratioof the respeC- Pauling General chemlstryv 1958 680*682' tive degrees ofchange in said two properties during said MORRIS Q WOLK Primary Examinertime interval. E I

6. The method of labeling a material which comprises 10 KATZ, lExaminer.

adding a clathrate compound in a trace amount to the said material andthereafter sensing a distinctive property US of the said clathratecompound in themixture, '250 83

